Due to the complexity and scale of the current Internet, large-scale simulations are an increasingly important tool to evaluate network protocol design. Parallel and distributed simulation is one appropriate approach to the simulation scalability problem, but it can require expensive hardware and have high overhead. In this study, we investigate a complimentary solution to large-scale simulation–-simulation abstraction. Just as a custom simulator includes only details necessary for the task at hand, we show how a general simulator can support configurable levels of detail for different simulations. We develop two abstraction techniques, centralized computation and abstract packet distribution, to abstract network and transport layer protocols. We demonstrate these techniques in multicast simulations and derives centralized multicast and abstract multicast distribution (session multicast). We show that our abstraction techniques each help to gain one order of magnitude in performance improvement (from tens to hundreds to thousands of nodes). Although abstraction simulations are not identical to more detailed simulations, we show that in many cases these differences are small. We show that these differences result in minimal changes in the conclusions drawn from simulations in reliable multicast simulations.